Synthetic linear hexamethylene adipamide polyamide yarns (often referred to as nylon 66) recently celebrated their 50th anniversary. An important use of such yarns is as textured multifilament yarns, e.g. for making apparel, such as hosiery. For many purposes, it is the high bulk that is desired in the textured yarns. For some years now, these bulky textured yarns have been prepared commercially in 2 stages; in a first process, nylon polymer has been melt spun into filaments that have been wound up into a (yarn) package at high speeds (of the order of 3000 meters per minute (mpm), so-called high speed spinning) as partially oriented yarn (sometimes referred to as POY) which is a feed yarn (or intermediate) for draw-texturing (and sr sometimes referred to as DTFY for draw-texturing feed yarn); then, in a separate process, the feed yarns have been draw-textured on commercial texturing machines. These processes have been described in several publications, e.g. by Adams, in U.S. Pat. No. 3,994,121, issued 1976. Draw-texturing of various types of POY has been practiced commercially for more than 10 years on a very large scale. This has encouraged improvement of texturing machines. Accordingly, texturing machines have for some time had speed capabilities of well over 1000 mpm. But it has proved too difficult to obtain the desired bulky nylon 66 yarns at such high speeds, mainly because of limitations in the nylon POY that has been commercially available. So, in the U.S.A., for preparing the bulky nylon yarns that have been desired, nylon POY has for some years been textured commercially at speeds well below even 1000 mpm, i.e., well below the capability of the texturing machines, which could have been operated at significantly higher speeds.
Recently, Chamberlin et al in U.S. Pat. Nos. 4,583,357, and 4,646,514 have discussed such yarns, and their production via partially-oriented nylon (referred to by Chamberlin as PON). The disclosures of these "Chamberlin" Patents are incorporated herein by reference as background to aspects of the present invention.
Chamberlin discloses an improved (PON) spinning process and product by increasing the molecular weight of the nylon polymer well above the levels previously customary for apparel end uses. The molecular weight of nylon yarn was measured by relative viscosity (RV) determined by ASTM D789-81, using 90% formic acid. The apparel yarns were of nylon 66 of denier between 15 and 250; this denier range for apparel yarns is in contrast to that used for nylon carpet yarns, that have been made and processed differently, and are of different (higher) deniers, and some such carpet yarns had previously been of higher RV than for nylon apparel; Chamberlin mentions the expense and some difficulties of using higher RVs than conventional when making apparel yarns. Chamberlin's higher RVs were greater than 46, preferably greater than 53, and especially greater than 60, and up to 80 (for nylon 66). Chamberlin compared the advantages of such yarns over yarns having a nominal polymer RV of 38-40. Chamberlin discloses preparing PON by spinning at high speeds greater than 2200 mpm, and as high as 5000 mpm. Chamberlin describes how his high RV high-speed spun PON feed yarns were draw-textured at 750 or 800 mpm on a Barmag FK6-L900 texturing machine using a 21/2 meter primary heater at 225.degree. C. and a Barmag disc-aggregate with Kyocera ceramic discs, at a D/Y ratio of about 1.95. (As indicated by its name, the Barmag FK6-L900 texturing machine is itself capable of operation at 900 meters/minute, i.e. at speeds higher than disclosed by Chamberlin; texturing machines that are capable of operating at even higher speeds have been available commercially for several years). Chamberlin obtained crimp development values that were better than for 40 RV conventional yarn without excessive broken filaments (frays), or yarn breaks under these conditions.
Chamberlin explained the operable texturing tension range, within which the draw ratio may be changed (at a given draw roll speed) by adjusting the feed roll speed and so the draw-texturing stress or tension, which should be high enough for stability in the false-twist zone (to avoid "surging") and yet low enough to avoid (excessive) filament breakage. So adjustments were made to get maximum crimp development by operating with "maximum texturing tension" within this operable tension range. So, even if a feed yarn can be textured satisfactorily at a given speed and under other specified conditions, the operable texturing tension range may be quite narrow. A narrow texturing range (or "window") is commercially disadvantageous, as it limits the texturer.
This may be further understood by reference to FIG. 1, in which schematically texturing tensions are plotted against texturing speed. When one operates at a texturing speed V.sub.L, the average tension prior to twist-insertion (referred to as pre-disc tension T.sub.1) is shown by the large dot, but the actual along-end tension T.sub.1 is more accurately represented by a distribution of tensions; i.e., T.sub.1 .+-.-.DELTA.T.sub.1, where .DELTA.T.sub.1 represents approximately 3 times the standard deviation of the tension. Therefore, a stable texturing process requires that the minimum tension (T.sub.1 -.DELTA.T.sub.1), rather than the average pre-disc tension (T.sub.1), be sufficiently high to prevent surging. To increase the texturing speed from V.sub.L to V.sub.H, for example, by just increasing texturing speed (denoted as path A), would result in a condition wherein, although the average texturing tension might seem acceptable, the process would be unstable whenever T.sub.1 drops, so surging would occur. So, in practice, an increase in texturing speed is achieved by increasing the average T.sub.1 (see path B) by increasing the texturing draw ratio. Although such a higher draw ratio may avoid surging and so provide for a stable texturing process, the texturer may now obtain lower bulk, and may even experience broken filaments because of the increase in texturing tensions across the twist device. The post-disc tensions (T.sub.2) are usually greater than the pre-disc tensions (T.sub.1); in FIG. 1 this higher value is denoted by 2'. To increase bulk and eliminate broken filaments, the texturer must decrease T.sub.2 tensions from 2' to a lower point denoted by 2. This is usually achieved by increasing the relative disc-to-yarn speed ratio (D/Y) which slightly increases the pre-disc tensions (T.sub.1), but significantly decreases the post-disc tensions (T.sub.2) and, therefore, the T.sub.2 /T.sub.1 ratio. A concern with higher D/Y-ratios is increased disc wear and abrasion of the yarn. Another option is to increase texturing temperature, as the post-disc tension (T.sub.2) usually decreases more than the pre-disc tension (T.sub.1) as the temperature increases. This option, also, may be undesirable, as it will reduce the tensile strength of the "hot" yarn during twist insertion and increase the propensity for broken filaments.
This balancing of texturing draw ratio, the disc/yarn speed ratio, and the heater plate temperature is frequently referred to as the "texturing window" which narrows for a given texturing machine configuration with increasing texturing speed, as shown in FIG. 1; there are upper tension limits beyond which broken filaments occur, and even process breaks, and lower tension limits, below which surging occurs and poor along-end textured yarn uniformity.